<!DOCTYPE html>
<html>
<head>
<script>

function boom()
{
  var frameDoc = document.getElementById("f").contentDocument;

  var confusedNode = frameDoc.createTextNode("y");
  confusedNode.__proto__ = document.createTextNode("x");
  confusedNode.setUserData("key", "data", null);
  confusedNode.setUserData("key", "data", null);
}

</script>
</head>
<body onload="boom();">
<iframe src="data:text/html,1" id="f"></iframe>
</body>
</html>
